To meet higher capacity demands and to enhance user experience, cellular communications network are increasing the number of base stations employed. One approach for increasing the density of base stations is achieved by cell splitting macro cells in highly loaded geographical areas. Specifically, the macro cell may be split into multiple small cells in highly loaded geographical areas. These highly loaded areas may be considered traffic hotspots within the coverage area of the macro cell. This densification of the underlying support for the cellular network may allow radio resources to be reused. Additionally, because wireless devices may be closer to the serving base station, wireless devices may achieve higher bitrates.
Another approach for meeting high capacity demands is to employ a mixture of macro cells and small cells with overlapping coverage areas within the cellular network. This type of cellular network may be referred to as heterogeneous networks (HetNets). Such networks may be an important complement to macro cell splitting. One example includes a cellular network having clusters of pico cells within the macro coverage area to offload macro traffic. A pico base station provides service to a pico cell. Typically, a pico base station is a low power node (LPN) that transmits with low output power and covers a much smaller geographical area than a high power node, such as a macro base station. Other examples of low power nodes are home base stations and relays.
Though the presence of additional base stations increases system performance and improves user experiences, such networks are not without its disadvantages. One such disadvantage may be that the wireless devices served by the network may experience lower geometries. As a result, downlink inter-cell interference may be more pronounced and the achievable bit rates may be limited. To mitigate inter-cell interference, mitigation techniques have been employed to improve user performance. Such techniques may explore the structure of the physical layer transmission of the radio access technology.
One technique for mitigating inter-cell interference includes frequency separation between the different layers of the network. For example, the macro cell and pico cells may operate on different non-overlapping carrier frequencies and thereby avoid any interference between the layers. With no macro cell interference towards the under laid cells, cell splitting gains may be achieved when all resources may simultaneously be used by the macro cell and pico cells. However, a disadvantage of operating layers on different carrier frequencies may be resource-utilization inefficiency. For example, when activity levels in the pico cells are low, the network may be operated more efficiently using all carrier frequencies in macro cell and disregarding the pico cells. However, because the split of carrier frequencies across layers is typically done in a static manner, operation of the network may not be adjusted based on the activity levels in the pico cells.
Another related technique for efficient operation of a heterogeneous includes sharing radio resources on same carrier frequencies by coordinating transmissions across a macro cell and the pico cells. This type of coordination refers to as inter-cell interference coordination (ICIC) in which certain radio resources are allocated for the macro cells during some time period whereas the remaining resources can be accessed by the under laid cells without interference from the macro cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to using a split of carrier frequencies between the macro cell and the pico cells, the ICIC sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between a base station and the low power nodes. For example, in LTE, an X2 interface has been specified in order to exchange different types of information between network node and low power nodes. One example of such information exchange is that each low power node may be capable of informing other low power nodes and the base station that it will reduce its transmit power on certain resources. However, time synchronization between the low power nodes is required to ensure that ICIC across layers will work efficiently in the heterogeneous network. This is in particular of importance for time domain based ICIC schemes where resources are shared in time on the same carrier.
Interference mitigation may take place on the transmitter side, the receiver side, or on both sides. Interference mitigation on the transmitter side includes those methods that seek to coordinate the physical channel transmissions across cells to avoid severe interference. For example, an aggressor base station may occasionally mute its transmissions on certain radio resources in order for a victim base station to schedule interference sensitive wireless devices on radio resources with reduced interference.
LTE features that seek to coordinate transmissions, on the network side, have been specified in the context of inter-cell interference coordination (ICIC) and coordinated multipoint transmissions (CoMP). In the case of ICIC, for example, a network node such as an eNode B may send a message over the LTE inter-eNB interface (X2). The message may include coordination information that a receiving network node, such as another, may use when scheduling interference sensitive wireless devices. As such, competing transmissions may be coordinated to avoid inter-point interference. As another example, CoMP may use a cluster of transmission points, or base stations, to jointly and synchronously transmit the same signals and thereby increase the received power on the desired signals.
The following ICIC messages over X2 have been specified in TS 36.423:                Uplink (UL) Overload Interference Indication (OII) indicates the interference level (low, medium, high) per resource block (RB) experienced by the indicated cell on all RBs.        UL High Interference Indication (HII) indicates the occurrence of high interference sensitivity per RB, as seen from the sending.        Received Narrow Transmit Power (RNTP) indicates per RB whether DL transmission power is lower than the value indicated by a threshold.        Almost Blank Subframe (ABS) pattern indicates the subframes on which the sending will reduce power for some physical channels and/or reduced activity.        
The X2 messages OII, HII and RNTP represent methods for coordinating physical data channel transmissions in the frequency domain across cells. In contrast, the ABS message is a time domain mechanism to primarily protect reception of PDCCH, PHICH and PDSCH in the small cells. ABS allows macro cells to occasionally mute or reduce transmit power on PDCCH/PDSCH in certain subframes. The ensures backwards compatibility towards wireless devices by continuing transmission of necessary channels and signals in the ABS pattern for acquiring system information and time synchronization.
On the receiver side, advanced receivers employing enhanced interference suppression schemes, maximum likelihood techniques and interference cancellation techniques are gaining popularity. Such advanced receivers operate to mitigate downlink (DL) interference arising from neighbor-cell transmissions to wireless devices in neighboring cells. Specifically, such receivers may explicitly remove all or parts of the interfering signal.
Generally, such receivers may be categorized into 3 families:                Linear receivers whose aim is to suppress the interference by exploiting an explicit channel estimation of the interfering signal(s).        Non-linear receivers such as ML detection (iterative or non-iterative).        Non-linear receivers such as Interference Cancellation (IC) receivers which explicitly cancel the interference from the received signal. IC receivers may be iterative or non-iterative.        
One specific type of receiver may use interference rejection combining (IRC) for mitigating inter-cell interference. IRC is a technique for suppressing interference, which requires estimation of an interference/noise covariance matrix. Another type of receiver for mitigating interference may include interference cancellation (IC) receivers that operate to estimate unwanted signals (intra/inter-cell interference). As an example, an IC receiver in the victim wireless device may operate to demodulate and optionally decode the interfering signals, produce an estimate of the transmitted and the corresponding received signal, and remove that estimate from the total received signal to improve the effective signal-to-noise ratio (SINR) for the desired signal. In post-decoding IC receivers, the interfering data signal is demodulated, decoded, its estimated contribution to the received signal is regenerated, and subtracted. In pre-decoding receivers, the regeneration step is performed directly after demodulation, bypassing the channel decoder. The preferred mode to perform such cancellation may include applying soft signal mapping and regeneration rather than hard symbol or bit decisions. Additionally or alternatively, maximum likelihood receivers may be used to jointly detect the desired signals and the interference signals in accordance to the maximum likelihood criterion. Iterative maximum likelihood receivers may be defined to exploit the decoding of the interfering signals.
Both IRC and IC are wireless device reference receiver techniques in LTE. However, IC in LTE is currently restricted to cancellation of always-on signals, such as the CRS, in which the network assists the wireless device on how these signals are transmitted in the aggressor cells. The two interference cancellation approaches differ by the achievable cancellation efficiency. Stated differently, the fraction of the impairment power left after the cancellation operation may be essentially equal in some scenarios and vary significantly in others. While the post-decoding IC approach may provide superior performance at “high” SIR operating points, these approaches have differing computational resource requirements. For example, the described post-decoding solution implies turbo decoding processing. Additionally, the processing delay incurred may vary by technique. For example, the post-decoding solution requires buffering the entire code block of the interfering signal.
To apply these advanced interference cancellation techniques to signals originating from other cells, knowledge of certain signal format parameters may be required to configure the receiver. For pre-decoding IC, the resource allocation, modulation format, any pre-coding applied, the number of layers, etc. may be useful, and may be obtained via blind estimation, eavesdropping other-cell control signaling, or via NW assistance features. For post-decoding, additional transport format parameters are required which may typically only be obtained from receiving or eavesdropping the related control signaling.
However, the different types of receivers may require differing information and/or parameters and are required to estimate blindly all the parameters needed for the receiver implementation. Additionally, the multitude of communication standards applicable to LTE may include many features which may need to be supported by the wireless device but which will not be used by a network (depending on the configuration) and may make blind detection difficult and complex. Currently no signaling is defined in LTE standard in order to provide wireless devices with the assistance which may be needed in order to implement advanced receivers with limited complexity.